Method of making spiral tube mixing device

ABSTRACT

High speed, very uniform mixing of two fluids is achieved by a mixing device formed from a pair of tubular members. The inner tube, which may be formed of sintered stainless steel powder, is porous along its entire length, but plugged at one end so that a first fluid under pressure within it will be forced radially outwardly through the pores. The outer tube is spirally corrugated and positioned in concentric relation with the porous tube. The inner surfaces of the corrugations contact the porous tube and cause a second fluid flowing from one end to the other of the corrugated tube to flow along the spiral path of the corrugations. Since the spiral corrugations cause the second fluid to flow very rapidly over the outer surface of the porous tube, the second fluid will continually strip or shear away the fluid exiting through the pores and thus promote extremely rapid and thorough mixing of the first fluid into the second fluid to form a mixed fluid or a reaction product, depending on the nature of the fluids in the two tubes. The tubes are placed in intimate contact with each other by an improved forming method wherein the outer tube is corrugated in two steps while in contact with the inner tube.

United States Patent [191 Thorne et a1.

[ Nov. 4, 1975 METHOD OF MAKING SPIRAL TUBE MIXING DEVICE [73] Assignee:Universal Oil Products Company, Des Plaines, Ill.

[22] Filed: Sept. 25, 1974 [21] Appl. No.: 509,154

Related U.S. Application Data [62] Division of Ser. No. 402,871, Oct, 2,1973, Pat. No.

[52] U.S. C1. 29/455; 29/456; 29/516; 29/520; 72/77 [51] Int. Cl. B21D39/00; B23P 11/00 [58] Field of Search 29/456, 516, 455, 520; 72/77;259/4; 62/513; 260/68348; 23/269; 165/156 [56] References Cited UNITEDSTATES PATENTS 531,000 12/1894 Morrison 29/455 UX 1,853,561 4/1932Hauton .1 29/516 UX 2,137,044 11/1938 Dawson 29/456 X 2,157,252 5/1939Visser et a1. 29/516 UX 3,040,760 6/1962 Macks I 29/455 UX 3,128,8214/1964 Andersen.. 72/77 3,566,615 3/1971 Roeder 62/513 X 3,568,4893/1971 Tobita 72/77 3,583,189 6/1971 Kelstrom 72/77 FOREIGN PATENTS ORAPPLICATIONS 682,453 11/1952 United Kingdom 29/456 PrimaryExaminer-Charlie T. Moon Attorney, Agent, or Firm-James R. Hoatson, Jr.;Barry L. Clark; William II. Page, I1

[ ABSTRACT High speed, very uniform mixing of two fluids is achieved bya mixing device formed from a pair of tubular members. The inner tube,which may be formed of sintered stainless steel powder, is porous alongits entire length, but plugged at one end so that a first fluid underpressure within it will be forced radially outwardly through the pores.The outer tube is spirally corrugated and positioned in concentricrelation with the porous tube. The inner surfaces of the corrugationscontact the porous tube and cause a second fluid flowing from one end tothe other of the corrugated tube to flow along the spiral path of thecorrugations. Since the spiral corrugations cause the second fluid toflow very rapidly over the outer surface of the porous tube, the secondfluid will continually strip or shear away the fluid exiting through thepores and thus promote extremely rapid and thorough mixing of the firstfluid into the second fluid to form a mixed fluid or a reaction product,depending on the nature of the fluids in the two tubes. The tubes areplaced in intimate contact with each other by an improved forming methodwherein the outer tube is corrugated in two steps while in contact withthe inner tube.

5 Claims, 5' Drawing Figures U.S. Patent Nov. 4, 1975 Sheet 1 of 23,916,504

\ Smut BACKGROUND OF THE INVENTION The invention relates to the field offluid mixing devices. Examples of prior art fluid mixers can be found inUS. Pat. Nos. 2,951,061; 3,240,337; 3,435,092; 3,438,720; 3,554,228; and3,677,714 where fluid is passed through openings or small pores in onemember into contact with a fluid in another member. In each of theseprior art devices the mixing is somewhat slow and non-uniform. Otherprior art devices, as exemplified by U.S. Pat. Nos. 2,908,486;3,158,192; 3,270,806; 3,498,370; 3,545,063; 3,566,615; and 3,709,665teach that heat transfer to or from a single fluid or between differentfluids can be enhanced by lengthening the contact path by causing atleast 'one of the fluids to follow a spiral or other tortuous path.These latter devices do not provide for any mixing of fluids.

Although many of the'prior art mixers provide fairly uniform mixing,there are certain mixing applications wherein it is desirable that thedispersion of one fluid into another be extremely rapid and that thedistribution of the fluids be uniform. Such an application might be inan alkylation reactor such as disclosed in US. Pat. Nos. 2,951,061;3,435,092 and 3,607,970 where an olefin-isoparaffin hydrocarbon streamis combined with a liquid acid stream.

SUMMARY OF THE INVENTION It is among the objects of this invention toprovide a mixing or reacting device for fluids which will promoteextremely rapid and uniform dispersion of one fluid into another, willprevent localized feed concentration buildups, will provide plug flowand prevent back mixing, and will be very compact.

These and other objects are achieved by themixing apparatus of thepresent invention in which a porous metal tube formed of a sinteredmetal powder, such as stainless steel, is closed or restricted at oneend and mounted within a surrounding spirally corrugated tube. A firstfluid is introduced into the inner porous tube under sufficient pressureto cause it to move radially outwardly through the pores in the tubewall. A second fluid, which is introduced into the corrugated tube at alower pressure, is forced to flow around the spiral path defined by theinner walls of the corrugations and the outer wall of the porous tubewhich the corrugations engage. Since the second fluid continually wipesthe surface of the porous tube, any fluid exiting from the pores in theporous tube will be sheared off and 'will instantly and uniformly bedispersed in the second fluid or will react with it to form a reactionproduct. In a modification of the mixing apparatus, one end of theporous tube is provided with a back pressure means, such as a springloaded check valve, which permits'a portion of the flow in the innertube to be recycled or transported to another location.

The apparatus provides the most efficient mixing when the corrugationson the outer tube are in intimate contact with the porous tube so thatthe fluid flowing between the tube walls will be forced to follow ahelical path. It has been found, however, that it is impossible totelescope together a porous tube and a corrugated tube when the outerwall of the porous tube and the 2 inner wall of the outer tube are ofthe same diameter or even within several thousandths of aninch of eachother without the metals seizing. The springback characteristics ofstainless steel during cold working have been found to prevent'a smoothoiiter tubefrom being corrugated into intimate contact' with a poroustube in the normal operation of a two die corrugating machine similar tothat disclosed in Andersen Pat. No: 3,128,821. We have found, however,that by making two passes of concentric porous and smooth tubes throughsuch a pair of dies, andrwitho ut changing the positions of the diesrelative to each other, it is possible to both form the corrugations intheouter tubeand force them into intimate contact with the porous tubeso that the corrugations on the outer tube will have a smaller internaldiameter than ,the outer diameter .of the porous inner tube immediatelyadjacent the spiral line of contact between the inner and outer tubes.Rather than make separate passes through a single pair of dies, it isalso possible to achieve intimate contact by passing the compositetubeassembly through successive pairs of dies or through a single set ofthree or more corrugating dies such as that disclosed in Kelstrom Pat.No. 2,583,189. v

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side sectional view of theimproved mixing device; i

FIG. 2 is a side sectional view similar to FIG. 1 but showing amodification; Y

FIG. 3 is a side sectional view showing a pair of cor-. rugating diescorrugating the outerwall of, the tube composite which forms the mixingdevice;

FIG. 4 is a viewidentical to FIG. ,3.except that the tube composite isshown undergoing a second pass through the corrugating .dies;-and

FlG.--5 is a view similar to FIG. 3 but showing a modified form ofcorrugating device.

DESCRIPTION OFITHE "PREFERRED EMBODIMENT Referring to FIG. 1, theimproved-.mixing device indicated generally at 10 includes a non-porousouter tube member 12 and a porous inner tube member 14. Each of thetubes 12, 14 ispreferably formed of stainless steel with the porousinner tube being formed bya sintering technique using metal powder. Thetube l2 has a non-corrugated exit end 16 and a non-corrugatedentranceend 18. The portion of the tube' 12 which is intermediate thenon-corrugated end portions 16, 18 includes corrugations which aredefined by a spiral ridge 20 which engages, and preferably, is slightlyembedded within the outer surface 21 of the porous tube 14. An inletfitting 22 welded to the side of outer tube 12 at end 18 thereof isinternally threaded to facilitate its connection to a source of fluid(not shown) to be passed through the tube 12. 1

The porous inner tube '14 has an internally threaded, non-porousinlet'fitting 26 welded toit. A spacing ring 28 fills the annular spacebetween the-tubes 12, 14 and is welded to each 'offt'he tubes by awe'ldbead 30. Alternatively, a flange couldzb'e formed'on the fitting 26which would serve the same purpose as ring 28. Flow of fluid through theinner end of the porous tube 14 is prevented by plug member 32 which iswelded to the tube. When the device 10 is in use, plug 32 blocks flowthrough the porous tube 14 so that the pressure of a first fluid passinginto the tube causes the fluid to flow through the myriad tiny openingsin the wall 34 of the tube. As the first fluid reaches the outer surface21 of the porous tube it is instantly sheared from said surface by asecond fluid flowing along the surface as it passes through spiralchannel 38 on its way from inlet fitting 22 to exit opening 16. Wherethe first and second fluids flowing through the inner and outer tubesare of a nature such as to produce an exothermic reaction upon contactwith each other, the disclosed mixing device is especially efficientsince mixing is extremely rapid and the respective fluids welldispersed. In addition, the extended outer surface of tube 12 producedby the corrugations enhances the ability of the device to dissipateexcess heat as it is produced. An example of such a reaction would bethe feeding of a hydrocarbon stream such as olefin-isoparaffin into theporous tube 14 and the outward radial dispersion of said stream into ahydrofluoric acid stream passing through spiral channel 38.

FIG. 2 shows a modification 40 of the mixing device shown in FIG. 1. Themixing device 40 includes a non-porous outer tube 42, a porous innertube 44, noncorrugated end portions 46, 48, a spiral corrugated ridge50, a threaded fluid inlet fitting 52, a non-porous inlet fitting 56 forthe porous tube 44, and an annular spacing ring 58 attached to the outertube 42 and the fitting 56 by a weld bead 60. The mixing device 40differs from the devie 10 of FIG. 1 in that flow through the porous tube44 is not completely prevented at the end of the tube opposite itsentrance end. Rather, a back pressure means comprising an end fitting 64is provided. The fitting 64 includes a flange 66 welded to the outertube 42 at 68. Since the weld 68 and flange 66 effectively block theexit end of tube 42 a side mounted, internally threaded fluid outletmember 70 is provided. The fitting 64 further includes a valve seat 74,and a ball valve member 76 biased against the seat 74 by a spring 78.Fluid passing by the valve member 76 exits through a threaded outletfitting 84. By means of a screw type adjustment member 80 threaded inthe end of fitting 64 the amount of back pressure exerted by the ball 76on the fluid within the porous tube 44 can be closely controlled. Thisback pressure adjustment feature is useful where it is desired to permita portion of the fluid within the porous tube to pass through the mixingdevice without being mixed and either carried to another location orreturned to the fluid source. The valve assembly also permits thepressure, and thus the flow rate, of the fluid passing through the wallsof the porous tube to be controlled independently of variations in theinlet pressure from the fluid source, as long as the inlet pressure isin excess of that required.

FIGS. 3 and 4 illustrate a preferred method for assembling a porousinner tube and a non-porous outer tube to each other. Referring to FIG.3, a porous inner tube 88 is shown after it has been positioned within asmooth outer tube 90. The tube composite is fed from left to rightthrough a pair of dies 92, 94 which may be of the type disclosed inAndersen Pat. No. 3,128,821. As the left die 92 engages the tube 90 itforms a groove and a corresponding ridge 90a which is forced intocontact with the outer surface of porous inner tube 88. Since thematerial of the tube 90 is preferably stainless steel, one must contendwith a property known as spring back" which causes the groove and ridgeformed by die 92 at a to spring away from the inner tube 88 as shown at90b as the groove and ridge advance relative to the die. In FIG. 4, thecorrugated composite formed by the dies of FIG. 3 is shown being passedback through the same dies 92, 94. This second pass provides a secondcold working operation which causes the ridges 90b to assume a finalposition in contact with the inner tube 88 as shown at 90c. In each dieset, the major deformation is done by the initial die. In order to keepthe ends of the tubes uncorrugated the dies are moved radially into thetubes only where corrugations are desired. Preferably, the dies forcethe ridges about 0.002 0.004 inch into the surface of the porous tube88.

FIG. 5 shows an alternative method of assembling the outer and innertubes to each other which utilizes a triple ring corrugating device ofthe type shown in Kelstrom Pat. No. 3,583,189. A porous tube 104 and asmooth non-corrugated outer tube 106 are fed from left to right throughleft die 108, center die 110 and right die 112. The additional coldworking of the tube material which takes place due to the right hand die112 causes the ridge, which is formed as shown at 106a by die 108, andwhich springs away after forming as shown at 106b, to be movedpermanently into contact with the porous tube 104 as shown at 106C.

The details of the mounting structures for the various dies shown inFIGS. 3 5 have not been shown since they are explained in great detailin the aforementioned U.S. Pat. Nos. 3,128,821 and 3,583,189, thecontent of which is incorporated herein by reference.

We claim as our invention:

1. A method of forming a mixing device comprinsing the steps of:positioning a cylindrical porous tube inside a cylindrical, non-porousouter tube, said outer tube having an inner diameter which is largerthan the outer diameter of the inner tube; corrugating the outer tube toform a spiral groove by means of a pair of dies engaged with oppositeouter side portions of the tube, said dies being axially spaced fromeach other; and additionally corrugating said tube by contacting saidgroove with at least one die to deepen said groove and force the ridgeswhich define it into intimate contact with said porous inner tube.

2. A method in accordance with claim 1 wherein said step of additionallycorrugating said tube forces said ridges into said porous tube so thatthe inner diameter of the corrugations is less than the outer diameterof the porous tube immediately adjacent the spiral line of contactbetween said inner and outer tubes.

3. A method in accordance with claim 1 wherein said additionalcorrugating step is performed by a third die member positioneddownstream of said pair of dies.

4. A method in accordance with claim 1 wherein said additionalcorrugating step is performed by a second pass of the tubes between saidpair of (lies.

5. A method in accordance with claim 4 wherein the radial distancebetween each of said pair of dies and the axis of the tubes is notaltered before making said second pass.

1. A method of forming a mixing device comprinsing the steps of:positioning a cylindrical porous tube inside a cylindrical, nonporousouter tube, said outer tube having an inner diameter which is largerthan the outer diameter of the inner tube; corrugating the outer tube toform a spiral groove by means of a pair of dies engaged with oppositeouter side portions of the tube, said dies being axially spaced fromeach other; and additionally corrugating said tube by contacting saidgroove with at least one die to deepen said groove and force the ridgeswhich define it into intimate contact with said porous inner tube.
 2. Amethod in accordance with claim 1 wherein said step of additionallycorrugating said tube forces said ridges into said porous tube so thatthe inner diameter of the corrugations is less than the outer diameterof the porous tube immediately adjacent the spiral line of contactbetween said inner and outer tubes.
 3. A method in accordance with claim1 wherein said additional corrugating step is performed by a third diemember positioned downstream of said pair of dies.
 4. A method inaccordance with claim 1 wherein said additional corrugating step isperformed by a second pass of the tubes between said pair of dies.
 5. Amethod in accordance with claim 4 wherein the radial distance betweeneach of said pair of dies and the axis of the tubes is not alteredbefore making said second pass.